In the semiconductor industry, there is a continuing trend toward manufacturing integrated circuits (ICs) with a greater number of layers and with higher device densities. To achieve these high densities there have been, and continues to be, efforts towards reducing the thickness of layers, improving the uniformity of layers, reducing the thickness of devices and scaling down device dimensions (e.g., at sub micron levels) on semiconductor wafers. In order to accomplish such higher device packing densities, thinner layers, more uniform layers, smaller feature sizes, and smaller separations between features are required. This can include the thickness of gate oxide materials (e.g., SiO2), the width and spacing of interconnecting lines, the spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. The scaling-down of integrated circuit dimensions can facilitate faster circuit performance, and can lead to higher effective yield in IC fabrication by providing more circuits on a die and/or more die per semiconductor wafer. Such advantages are a driving force to constantly scale down IC dimensions.
The process of manufacturing integrated circuits typically consists of more than a hundred steps, during which hundreds of copies of an integrated circuit can be formed on a single wafer. Generally, the process involves creating several layers on and in a substrate that ultimately forms the complete integrated circuit. This layering process can create electrically active regions in and on the semiconductor wafer surface. In metal-oxide-semiconductor (MOS) transistors, for example, a gate structure is created, which can be energized to establish an electric field within a semiconductor channel, by which current is enabled to flow between a source region and a drain region within the transistor. The source and drain regions facilitate this conductance by virtue of containing a majority of p or n type materials. The regions are typically formed by adding dopants to targeted areas on either side of the channel region in a semiconductor substrate. The gate structure includes a gate dielectric and a contact or gate electrode. The gate contact generally includes metal or doped polysilicon and is formed over the gate dielectric, which is itself formed over the channel region. The gate dielectric is an insulator material, which prevents large currents from flowing from the gate electrode into the channel when a voltage is applied to the gate contact, while allowing an applied gate voltage to set up an electric field within the channel region in a controllable manner.
Transistors are physically very small in many cases, whereby many such devices may be formed on a single-crystal silicon substrate (which can include a base semiconductor wafer and any epitaxial layers or other type semiconductor layers formed thereover or associated therewith) and interconnected in an integrated circuit. Nevertheless, the size of the transistors and other electrical components is continually decreasing to improve device density. However, certain properties of the materials utilized to form the transistors limit the size to which the transistors can be reduced. By way of example, properties of silicon dioxide (SiO2), which is commonly used to form the layer comprising the gate dielectric in transistors, can limit the degree to which the thickness of the gate dielectric can be reduced. For instance, extremely thin SiO2 layers allow for significant gate leakage currents due to direct tunneling of charge carriers through the oxide. Thus, it has been found that operating parameters may change dramatically due to slight variations in gate dielectric thickness.
Furthermore, thin gate dielectric layers are known to provide poor diffusion barriers to impurities. Thus, for example, extremely thin SiO2 gate dielectric layers suffer from high boron penetration into the underlying channel region during doping of the source/drain regions. Consequently, recent efforts at device scaling have focused on alternative dielectric materials that can be formed in a thicker layer than silicon dioxide layers and yet still produce the same field effect performance. These materials are often referred to as high-k materials because their dielectric constants are greater than that of SiO2. The relative performance of such high-k materials is often expressed as equivalent oxide thickness (EOT) because the alternative material layer may be thicker, while providing the equivalent electrical effect of a much thinner layer of SiO2. Accordingly, high-k dielectric materials can be utilized to form gate dielectrics, and the high-k materials facilitate a reduction in device dimensions while maintaining a consistency of desired device performance.